NHibernate Spatial (Part 1 - Introduction)

Part 1 - Introduction
Part 2 - First Demo
Part 3 - Improving
Part 4 - Querying
Part 5 - NHibernate 3.3.1+

NHibernate is a nice, tested and mature ORM. I've used it extensively since version 1.2 and it just keeps getting better and better.
It obviously provides many challenges, most of them regarding it's learning curve and some limitations, like handling spatial data at the database

Nowadays several Databases provide spatial extensions. The most notorious are probably PostGIS, Oracle Spatial and more recently SQL Server 2008. NHibernate does not natively handle this type of data but fortunately there's a library that fills this gap quite nicely: NHibernate Spatial Extensions.

In this post (or series of posts) I'm going to do a complete tutorial on setting up a SQL Server 2008 R2 database with spatial columns, mapping the tables to domain objects using NHibernate Spatial and querying them with HQL and QueryOver.

  Although NHibernate Spatial Extensions is a great project, it's being developed in "short bursts". Looking at the SVN repository we see active development through:

• August 2008 - October 2008
•  February 2009 - July 2009 
• October 2010 - December 2010 
• and finally, almost a year later, December 2011

The binary version available to download is from NHibernate 2.1 so getting the code from NH Contrib SVN repository and building it is almost a requirement.

First step:

• Checkout the code from: https://nhcontrib.svn.sourceforge.net/svnroot/nhcontrib/trunk/src/NHibernate.Spatial

• Open the solution NHibernate.Spatial.vs2008.sln

• Build it

WTF? The build fails?

I appreciate the tribute to my blog, but let's fix this.

The error states that:



I know for a fact that this project used to compile correctly so, looking at the recent commits I see this:

The extension method IsNull was removed on the changeset 1688. I don't really understand why, but I'll add it again.

Copy&Paste friendly:

/// <summary>
/// A fully compatible null checking. Use instead of " == null " expression.
/// </summary>
/// <remarks>
/// <para>
/// Using an equality to null lambda expression throws an exception in SQL Server
/// ("Invalid operator for data type. Operator equals equal to, type equals geometry.")
/// because NHibernate is generating an HQL expression like this:
/// </para>
/// <code>
///     (t.geom is null) and (null is null) or t.geom = null
/// </code>
/// <para>
/// Using this extension method, we generate just the following HQL:
/// </para>
/// <code>
///     t.geom is null
/// </code>
/// </remarks>
/// <param name="geometry"></param>
/// <returns></returns>
public static bool IsNull(this IGeometry geometry)
{ return geometry == null; }

Build again and it should... fail?

 After a quick inspection these classes are referenced from the NetTopologySuite dll. I went to the NetTopologySuite project page to download the version 1.10 of the library and saw this in the downloaded zip:

My spider senses are tingling. It seems that the good folks of the NetTopologySuite split the DLL with the provider specifics (which makes sense), the NH Spatial guys updated the NetTopologySuite reference, but didn't include the extra DLLs in the MsSql and PostGis projects. I don't really understand how this happened... Anyway, let's add the missing references:

Project NHibernate.Spatial.MsSqlSpatial.vs2008 -> add reference to NetTopologySuite.IO.MsSqlSpatial.dll

Project NHibernate.Spatial.PostGis.vs2008 -> add reference to NetTopologySuite.IO.PostGIS.dll

Build... Failed

Well, let's hope this is the last error.

Project Tests.NHibernate.Spatial.MsSql2008.vs2008 -> add reference to GeoAPI.dll

Build Succeeded.

I had planned to advance a little more in this post, but I'll wrap it up here. In my next post I'm going to create a very simple application that connects to a SQL Server 2008 database with Geography columns.

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Go to part 2